1. Field of the Invention
The present invention relates to an apparatus and method for making inter-vehicle communication, which enables an inter-vehicle communication apparatus that has transmitted a data packet to detect collision of the data packet with other data packets transmitted from other inter-vehicle communication apparatus.
2. Description of the Related Art
Inter-vehicle communication apparatus have been known which are mounted in vehicles to make communication between vehicles. For example, Japanese Patent Application Laid-Open No. 2003-258715 discloses an inter-vehicle communication apparatus for vehicles, which is adapted to detect the number of vehicles around the vehicle of its own, randomly determine a transmission interval based on the number of the vehicles and transmit a frame containing vehicle information at the determined interval. Also, such an inter-vehicle communication apparatus is adapted to randomly determine a transmission interval based on a travel speed corresponding to the externally inputted travel speed information of the vehicle of its own, and transmit a frame containing the vehicle information at the determined transmission interval. In this way, the collision frequency between communication packets is believed to be reduced and to enhance the communication efficiency.
However, in an inter-vehicle communication apparatus, as mentioned above, for use in wireless communication between vehicles, the vehicle, per se, that has transmitted a communication packet cannot know the fact of the occurrence of collision. As a measure for this, inter-vehicle communication apparatus are allowed to transmit/receive response signals (i.e., acknowledgement, which is hereinafter referred to as ACK) between them, which signals indicate normal reception of the communication packet. In this way, a vehicle can indirectly detect the occurrence of the collision between communication packets. If a collision has occurred, for example, the communication packet in question can be retransmitted as required. It should be appreciated that, in case of so-called one-on-one communication, a communication link established in advance may enable designation of other vehicles to communicate with. In this case, since the inter-vehicle communication apparatus of those vehicles which have not been designated may never respond, no collision may occur between communication packets.
Taking an example, an explanation will be provided below on a method for providing a location of a vehicle of interest. Two methods can be cited as the methods for providing a location of the vehicle of interest. One method is performed by making transmission in response to an interrogation from the other vehicle or a roadside machine, and the other method is performed by making transmission on a periodic basis.
The former method of responding to an interrogation limits a communication area and is effective where there are only a few vehicles in the communication area. When several hundreds of vehicles are in the communication area, such as in a traffic jam, these vehicles may respond to the interrogation all at once. Collision between response packets therefore is unavoidable and thus no normal communication can be expected. For this reason, it is preferable that transmission of the information on the vehicle location may be periodically performed and that the nearby vehicles may receive and utilize the information.
In one particular example, time required for transmission per packet is set as being 1 millisecond and the transmission period is changed between traffic-jam traveling and high-speed traveling, so that the packet density can be controlled so as not to be exceedingly high. For example, the time required for transmission per packet may be set at one second in traffic-jam traveling and 100 milliseconds for high-speed traveling. This may enable communication with a maximum of 1000 vehicles in the communication area in a traffic-jam and maximum of 100 vehicles in high-speed traveling.
The inter-vehicle communication apparatus used for inter-vehicle wireless communication described above, however, may establish a one-way communication mode, one application of the inter-vehicle communication, such as the case of notifying a location of the vehicle of interest, in which no response (ACK) as mentioned above may be made. It has thus been a problem that, under such circumstances, difficulty arises in determining the occurrence of collision between communication packets.